A dual mobility cotyloid implant includes, in a known manner, on one hand a cup intended to be fastened in a cotyloid cavity and including a substantially hemispherical polar portion and an equatorial portion extending the polar portion, said polar portion and the equatorial portion delimiting a substantially hemispherical inner cavity, and on the other hand an insert pivotally mounted in the inner cavity delimited by the polar and equatorial portions of the cup, the insert being outwardly delimited by an outer spherical surface portion and itself delimiting an inner cavity having an inner spherical surface portion and intended to pivotally and retentively assemble a prosthetic femoral head.
The present of such an equatorial portion makes it possible to limit the risks of an insert leaving the corresponding cup, and therefore the risks of luxation of the implant. Such a cotyloid implant therefore limits repeated surgeries.
It must, however, be noted that the risks of luxation remain significant when the “jumping distance,” i.e. the distance separating the bottom of the cup and the edge delimiting the opening thereof, is small, which is normally the case for the small implant sizes.
In order to limit the risks of luxation for small implant sizes, it is known to provide a substantial and constant equatorial portion height for all of the implants belonging to a line so as to ensure a satisfactory jumping distance for all of the implants.
However, the equatorial portion of the cup of such a cotyloid implant may, for large implants, come into conflict with the muscles or tendons situated close to the acetabular bone, and in particular with the patient's psoas, and therefore cause pain for the patient.
In order to reduce the risks of luxation, it is also known to offset the centers of the outer and inner spherical surface portions of the insert so that the center of the inner spherical surface portion is situated on the bottom side of the inner cavity delimited by the insert. These arrangements make it possible to ensure optimal repositioning of the insert relative to the cup in the upright position of the patient, due to the torque created by the forces exerted on the prosthetic femoral head, which causes the insert to rotate until it is in the balanced position.
It must, however, be noted that the offset between these two centers may not be sufficient to ensure optimal repositioning of the insert for large implants. Indeed, the larger the implant, the more the friction between the outer surface of the insert and the inner surface of the cup are significant and thereby oppose the torque created by the forces exerted on the prosthetic femoral head and aiming to replace the insert, which can cause a risk of luxation of the femoral prosthetic head and thus damage the reliability of the implant.